grownup vs full-grown

grownup is a noun but full-grown is not a noun.

Word	Noun	Adjective	Verb	Adverb
grownup	Yes	Yes	No	No
full-grown	No	Yes	No	No

As adjectives, grownup and full-grown are synonyms defined as:

Other synonyms of grownup include adult, big, fully grown, grown.

grownup (noun)	full-grown (noun)
a fully developed person from maturity onward

grownup (adjective)	full-grown (adjective)
(of animals) fully developed	(of animals) fully developed